Experimental melting of spinel lherzolites and the problem of the primary magma genesis of oceanic basalts

Melting of two spinel lherzolites chemically similar to the source of mid-o cean ridge basalts was studied experimentally at 3.5-20 kbar and 1200-1450 degrees C. A modified sandwich method was used with the lowest melting mine rals of peridotite (spinel and clinopyroxene) in the central part of a Iher zolite sample, which resulted in the generation of large melt pools appropr iate for microprobe analysis already at a melt fraction of 10%. The lherzol ites studied showed similar character of melting: clinopyroxene was the fir st mineral to disappear 40-60 degrees C above the solidus, while orthopyrox ene and spinel were preserved up to melt fractions of 30-40%. SiO2, MgO, Ca O, and FeO contents do not depend on the composition of starting peridotite at low degrees of melting and are controlled by pressure and temperature. The comparison of glass compositions from midocean ridge basalts with the e xperimental melts showed that the primary magmas of these basalts contained 12-18 wt % MgO. Such melts could be produced from a slightly depleted peri dotite with the predominant role of liquids derived at pressures of 10-18 k bar and melt fractions of 20-25%. Melts formed at higher pressures give ris e to basalts with higher FeO at a given MgO content. Some variations in the compositions of basaltic glasses suggest probable heterogeneity of their m antle source.